This invention generally relates to signalling devices. More specifically, it relates to a signalling device that produces an alarm whenever the distance between a transmitter and a receiver exceeds a predetermined distance.
There are a number of applications for such signalling devices. For example, a person in the confusion of travelling might leave luggage in a terminal. It would be helpful to alert the person that his luggage has been left so he could avoid the inconvenience of lost luggage. Examples of other applications include alerting a parent if a child were to wander from some predetermined area, or permitting a pet owner to monitor the whereabouts of a pet.
There are several prior anti-theft devices that typically include a transmitter and a receiver. The receiver measures the amplitude of the incoming signal and sounds an alarm when the amplitude falls below a predetermined level. In these applications, however, the receiver and alarm are placed on an article, such as an automotive vehicle, that is being protected so that the alarm sounds if the vehicle is driven more than a predetermined distance from a central location, such as a garage, without disabling the receiver.
Even where the transmitter is located on the person or article being monitored or if the foregoing anti-theft devices were offered with installation of the transmitter on the person or article being monitored, the resulting system would still be prone to interference from spurious signals. More specifically, when the alarm is sensitive only to the amplitude of a detected signal an interfering signal could cause the detection circuitry to falsely sense the presence of the transmitted signal.